If you want to carry your DD1, I'd be wary of stretchy wraps, to be honest - I had a hug-a-bub with DD and it was fantastic when she was little, but by about 7 months it was really starting to struggle with her weight (stretched and didn't stay secure).
Have you ever used a wrap? A woven wrap would work, and they come in a variety of lengths to fit different carries and body shapes, but is a slightly steeper learning curve than a stretchy as the fabric is a little less forgiving - can be very supportive for both you and your LO once you get the hang of it though.
Or how about a Mei Tai? A bit easier to learn how to get DD securely on your back, but doesn't give you as many options or quite as much snuglability (IMO) for a newborn.
There are quite a lot of different Soft Structured Carriers (SSCs) out there, of which Ergos and Patapums are two of the more common examples - but there are lots of others and they do come up 2nd hand (Connecta's seem to get quite good press, but there are lots of others out there - not really my area as I never got on with buckles).
The only issue you might have with both MTs and SSCs is that if the carrier is big enough for your 2 yr old it may swamp your newborn - Ergo get round this by having a "newborn insert", Patapum and many of the others by making two different sizes. If you get something less structured it is possible to roll the waist to shorten it and do various other tweaks, but it makes things less straightforward.
Can you get to a slingmeet? It would be a good way to have a prod an poke of a few different types of carriers to see what you're comfortable with. If you want to look around at slings 2nd hand, Natural Mamas and UKbabywearingSWAP are both worth a gander. I'd be wary of getting a 2nd hand stretchy wrap after my experience, but I've bought all my other slings pre-used and they've all been absolutely fine.
Happy hunting